Not applicable.
The technology herein relates to systems using vertical cavity, surface emitting lasers (VCSELs), and more particularly to such VCSELs having integrated MEMS (micro-electromechanical) wavelength tuning means to interrogate optical sensors. Still more particularly, the technology herein relates to such systems for use in interrogating fiber and planar Bragg gratings and etalons sensitive to physical stimuli, and to specific system configurations for use with such Bragg grating and etalon sensing devices.
Fiber optic sensors employing measurement of the shift of wavelength position of a sensor""s spectral peculiarity (maximum, minimum or some other function) under the influence of a physical stimulus are well known to those skilled in the art. Examples of such sensors include Bragg grating-based strain, pressure, temperature and current (via the associated magnetic fields) sensors and Fabry-Perot (FP) etalon pressure, temperature and strain sensors to name a few. Unfortunately, the widespread use of such sensors has generally been restricted in the marketplace because of many well known problems, including the susceptibility of simple, inexpensive sensing systems to optical noise and the great expense of most of the solutions found to overcome said susceptibility.
We have, in contrast, discovered that combining a new type of laser, a vertical cavity, surface emitting laser (VCSEL), with an integrated microelectromechanical (MEMS) tuning mechanism as an interrogating instrument with sensors of many different types will enable new, less expensive and more reliable class of optical sensor systems.
As is well known, a Bragg grating is a series of optical elements that create a periodic pattern of differing indices of refraction in the direction of propagation of a light beam. A Bragg grating is generally formed in an optical fiber by means of exposing ultraviolet sensitive glass (usually germanium doped fiber) with an ultraviolet (UV) beam that varies periodically in intensity, usually accomplished by means of an interference pattern created by a phase mask or a split beam, such as with a Lloyd""s mirror apparatus. Planar Bragg gratings are created by exposing xe2x80x9cphotoresistxe2x80x9d of any of a number of types through a phase shift or other type of mask, or holographic exposure, or they can be written directly with an electron beam. Light reflections caused by the periodic index of refraction pattern in the resulting grating interfere constructively and destructively. Since the refractive index contrast between UV-exposed and unexposed sections of fiber is small but the number of sections is very large, the reflected beam narrows its spectrum to a sharp peak, as narrow as a fraction of a nanometer in spectral width. In addition, the phase spectral dependences of the reflected and transmitted light generally exhibit some modification around the wavelength of said reflection peak.
It is known that Bragg gratings patterned into optical fibers or other waveguides may be used to detect physical stimuli caused by various physical parameters, such as, for example, strain, pressure, temperature, and current (via the associated magnetic fields) at the location of the gratings. See for example U.S. Pat. Nos. 4,806,012 and 4,761,073 both to Meltz, et al; U.S. Pat. No. 5,380,995 issued to E. Udd; U.S. Pat. No. 6,024,488 issued to J. Wu; and the publication authored by Kersey, A. D., et.al. [10th Optical Fiber Sensors Conference, Glasgow, October 1994, pp.53-56]. Generally, in such a sensor, the core and/or cladding of the optical fiber (or planar waveguide) is written with periodic grating patterns effective for selectively reflecting a narrow wavelength band of light from a broader wavelength band launched into the core (waveguide layer in the waveguide). The spectral positions of sharp maxima or minima in the reflected or transmitted light intensity spectra indicate the intensity of strain, temperature, pressure, electrical current, or magnetic field variations at the location of the grating. The mechanism of the spectral position variability lies in changes in either the grating period or the indices of refraction, or both, which can be affected by various environmental physical stimuli, such as, for example, temperature and pressure. Frequently, more than one stimulus or physical parameter affects the sensors at the same time, and compensation must be designed into the sensor or the measurement technique for all the variables but one (which can be accomplished by many physical, optical and electronic techniques known in the art). The typical sensitivity limits of fiber grating sensors in the current art are generally about 0.1xc2x0 C. to 1xc2x0 C. and/or 1 microstrain or higher (depending on the packaging and/or embedding of the sensor), respectively. Advantages of a spectral shift method of sensor interrogations include the high accuracy of wavelength determination (akin to the advantages of measuring frequency instead of magnitude) and immunity to xe2x80x9coptical noisexe2x80x9d due to fluctuations in fiber transmission amplitude (microbending losses, etc.). The use of Bragg gratings also allows the multiplexing of many sensors on the same fiber via wavelength dependent multiplexing techniques (WDM), e.g., dividing the total wavelength band into sections dedicated to individual sensors.
Another approach for the interrogation of fiber Bragg grating strain sensors has been disclosed by M. E. Froggatt, (U.S. Pat. Nos. 5,798,521 and 6,566,648, articles [Froggatt M., xe2x80x9cDistributed measurement of the complex modulation of a photoinduced Bragg grating in an optical fiberxe2x80x9d, Applied Optics, 35 (25), pp. 5162-5164, September 1996] and [Froggatt M., Moore J., xe2x80x9cDistributed measurement of static strain in an optical fiber with multiple Bragg gratings at nominally equal wavelengthsxe2x80x9d, Applied Optics, 37 (10), pp. 1741-1746, April 1998]). This approach is based on an interferometric scheme (FP interferometer) utilizing a coherent optical source such as a continuously tunable laser with a very narrow wavelength range (over just 0.23 nm) and a discretely tunable laser (over 2.5 nm total). The Froggatt method utilizes Fourier transformation of the measured spectrum, filtering of the Fourier transform followed by inverse Fourier transformation. Such an approach permits the acquisition of phase information, which in turn permits the multiplexing of a large array of fiber Bragg gratings having the same wavelength position of their reflectance peaks (unlike WDM, where a different spectral position of the reflectance peak of each sensor is essential). Such a technique is known as Optical Frequency Domain Multiplexing (OFDM). Large numbers of multiplexed sensors (up to 22) have been demonstrated [Froggatt M., Moore J., xe2x80x9cDistributed measurement of static strain in an optical fiber with multiple Bragg gratings at nominally equal wavelengthsxe2x80x9d, Applied Optics, 37 (10), pp. 1741-1746, April 1998]. However, such a technique also may suffer from significant limitations. First, the nature of the laser used can make the detection scheme complex due to the necessity of using complex requirements for wavelength determination. Second, the accuracy and resolution of the instrument may be far from optimal due to the limited wavelength range of the laser that was specified. Third, the update rate of such an instrument may be quite slow due to both the slow tuning speed of the laser specified and the large computational overhead from the active wavelength determination scheme used, which in turn limited the accuracy of the instrument. Fourth, the detection range of these systems may be limited by the short coherence length of the laser. Fifth, the price of such a system may be very high compared to competitive electronic techniques, due both to the laser and the active wavelength determination scheme used. Despite the attractiveness of the Froggatt approach, it may be stated that this scheme has not reached wide market acceptance.
The precision, dynamic range and multiplexing capabilities of the alloptical sensor interrogation techniques reviewed above, other than OFDR, are generally defined in part by the spectral power of the light source, especially in cases in which a broadband source is used. The LEDs, SLDs (superluminescent diodes) and various lamps usually used provide spectral power that can be too little when divided into subnanometer-sized segments (average power divided by wavelength range). This limits critical parameters such as the magnitude of the reflected peak available to the optical sensor, causing lower-than-desirable signal-to-noise ratios. Another technique, the use of a conventional laser diode tuned with a motorized external cavity, electrical current or temperature mechanisms tends to be more effective because all the power of the laser is contained in a narrow beam as it is tuned across the spectrum. Several techniques have been proposed. One is the use of a conventional laser diode tuned with electrical current, which has been proposed by Dunphy et. al. (U.S. Pat. No. 5,401,956). Another is the use of a tunable fiber laser, which has been proposed by G. A. Ball et. al. [J. of Lightwave Technology, vol. 12, no. 4, April 1994 p 700]. When using a scanning laser technique, an inexpensive detector and electronics system simply and easily determines the wavelength at the peak (or null) of the reflected (or transmitted) light intensity against a known wavelength reference. However, past art approaches are generally too expensive, too slow, too unstable or too inaccurate to have a wide range of practical applications. Laser diodes tuned with their excitation current, while inexpensive and faster than thermal methods, suffer from narrow tuning wavelength spans and changing optical power, which may limit practical applications to only time division-multiplexed (TDM) Bragg sensors. The broadband light source method utilizes an inexpensive light source, but generally requires a spectrometer to read the signals (an optical spectrum analyzer may cost as much as $35,000). The broadband method is most practical when many sensors are multiplexed on the same fiber. Still, spectrometers are temperamental and not well suited to field use. The lasers tuned with external cavities that are now in use, on the other hand, typically are more expensive than spectrometers, but have the advantage of using an inexpensive detector. In addition, such lasers are typically slow to tune, such as 100 nm/sec, and may be even more delicate than spectrometers. Scanning (or tuning) speed is especially important in applications in which absorption and polarization related noise are significant because of the degrading effects these noise sources on the signal to noise ration (SNR). One the other hand, mass-produced MEMS-tunable VCSELs, configured as sensing instruments, are expected to cost at least an order of magnitude less than prior art lasers and be at least two orders of magnitude faster than prior art lasers.
Fiber etalon-based sensors (also known as Fabry-Perot sensors) are well known to those skilled in the art (see, for example, U.S. Pat. No. 5,646,401 issued to E. Udd). Etalons consist of two mirrored surfaces that may be internal or external to the optical fiber. The reflectivity of an etalon is defined by interference between light waves reflected from first and second mirrors (or reflecting surfaces). The advantages of etalon-based pressure, temperature and/or stain sensors include the low cost of etalons and very high sensitivity. However, with broadband light sources used for interrogation, measurements that are based on light intensity or count interference fringes are very susceptible to optical noise or other technical problems (e.g., losing count of the fringes), to the point of being impractical. The sole practical, self-calibrating system uses an optical cross-correlating interferometer as a detector, also an expensive technique (see, for example, U.S. Pat. Nos. 5,202,939 and 5,392,117 both issued to Belleville, et al.). However, the multiplexing of a number of sensors with such a technique (such as required for structural monitoring and many other applications) is impossible as far as is presently known.
A new kind of laser, a vertical cavity surface emitting laser (VCSEL), has recently been developed. Generally, VCSELs are made completely with wafer-level processing and the chips emit from the direction of the broad surface of the wafer, rather than having to be cleaved out of the wafer in order to have an exposed p-n junction edge from which to emit, as in older art. This enables another benefit to be designed into the wafer structurexe2x80x94tunability. This is done with micromachining (MEMS) technology by placing a stack of optical layers, forming a mirror, in front of the emitting surface in such a way that the stack can be varied in its distance from the emitting surface by piezoelectric, magnetic, electrostatic or some other micro-actuating means. The groups of C. J. Chang-Hasnain (US Patent, [IEEE J. on Selected Topics in Quantum Electronics, V 6, N 6, November 2000, p. 978]), J. S. Harris Jr. (U.S. Pat. No. 5,291,502, [Appl. Phys. Lett. 68 (7), February 1996 p. 891]), and Vakhshoori [Electronics Letters, May 1999, V. 35, N.11 p. 900] have shown the potential for making tunable VCSELs with MEMS tuning mechanisms with wide tuning ranges and fast tuning speeds combined with good coherence length (exceeding 2 meters) and extreme reliability of the tuner mechanism (it survives hundreds of megacycles). Tunable VCSELs are relatively simple to manufacture, exhibit continuous mode-hop-free tunability over a wide spectrum, and offer more than an order of magnitude lower cost as compared to prior art tunable lasers or optical spectrometers. Integrated, MEMS-tunable VCSELS make possible truly affordable and accurate optical sensor systems by combining low cost detectors and low cost excitation sources, one or the other of which is very expensive in the prior art systems with the accuracy and resolution considerably exceeding those of the prior art lasers.
We use such VCSEL technology in a novel way to provide a means of optical wavelength scanning Bragg grating and etalon resonance sensors of all types with integrated, MEMS-tunable VCSELs in order to measure various physical parameters at several orders of magnitude lower cost than prior art, with the added benefits of enhanced accuracy, ruggedness and reliability.
In more detail, an exemplary illustrative non-limiting arrangement provides a diagnostic system which interfaces with optical fibers or optical waveguides having Bragg grating or other types of sensors as described herein, embedded therein for the determination of static and dynamic values of various physical parameters, and, further, to provide means of guaranteeing wavelength accuracy during the scanning cycle.
In accordance with an illustrative non-limiting aspect of an exemplary non-limiting illustrative implementation, an optical sensor diagnostic system includes an integrated MEMS-tunable VCSEL for providing a wavelength-tunable light in response to a voltage or other control signal, the tunable light being launched into an optical waveguide. At least one optical sensor, disposed in the path of the tunable light, provides a reflected light having an associated local amplitude and/or phase perturbation (for example, maximum or minimum). The wavelength at said minimum or maximum of amplitude varies in response to an environmental stimulus imposed upon the corresponding sensor. The tunable VCSEL individually illuminates each of the sensors throughout its associated wavelength band of an amplitude minimum or maximum. A reference fiber length (serving as a arm of an interferometer) is disposed between said VCSEL assembly and an optical sensor or sensor array. One or two reflection means are disposed in a reference fiber length to create a reference optical length in a fiber. A coupler or circulator must be provided to divert the optical signal reflected from the sensor array and the reflection means to the photodetector, the electrical signal from which is relayed to the control block circuitry and external electronic circuitry as required. The control block controls the laser temperature via a thermoelectric element or other means and may or may not adjust the laser power output according to a signal from a monitor phoitodiode, as required. A tuning controller provides a variable voltage or other signal to the tunable VCSEL indicative of the desired wavelength of the tunable light. A signal processor responsive to the electrical detection signal interprets a shift in the wavelength position of the magnitude minimum or maximum due to the environmental stimulus calculated from the recorded electrical detection signals, and provides a signal indicative of said stimulus.
In accordance with another illustrative aspect of an exemplary non-limiting illustrative implementation, an optical sensor diagnostic system includes an integrated MEMS-tunable VCSEL for providing a wavelength-tunable light in response to a voltage or other control signal, the tunable light being launched into an optical waveguide. At least one optical sensor, disposed in the path of the tunable light, provides a reflected light having an associated local amplitude and/or phase perturbation (for example, maximum or minimum). The wavelength at said minimum or maximum of amplitude varies in response to an environmental stimulus imposed upon the corresponding sensor. The tunable VCSEL individually illuminates each of the sensors throughout its associated wavelength band of an amplitude minimum or maximum. A reference fiber length (serving as an arm of an interferometer) is disposed in a separate optical path from the path connecting said VCSEL assembly and an optical sensor or sensor array. One or two reflection means are disposed in a reference fiber length to create a reference optical length in a fiber. At least one coupler or circulator must be provided to divide the optical signal from the tunable VCSEL into the reference and sensing paths and to divert the optical signal reflected from the sensor array and from the reflection means in the reference path to the photodetector, the electrical signal from which is relayed to the control block circuitry and external electronic circuitry as required. The control block controls the laser temperature via a thermoelectric element or other means and may or may not adjust the laser power output according to a signal from a monitor photodiode, as required. A tuning controller provides a variable voltage or other signal to the tunable VCSEL indicative of the desired wavelength of the tunable light. A signal processor responsive to the electrical detection signal interprets a shift in the wavelength position of the magnitude minimum or maximum due to the environmental stimulus calculated from the recorded electrical detection signals, and provides a signal indicative of said stimulus.
In accordance with a further non-limiting aspect of an exemplary non-limiting illustrative, an optical sensor diagnostic system includes an integrated MEMS-tunable VCSEL for providing a wavelength-tunable light in response to a voltage or other control signal, the tunable light being launched into an optical waveguide. At least one optical sensor, disposed in the path of the tunable light, provides a reflected light having an associated local amplitude and/or phase perturbation (for example, maximum or minimum). The wavelength at said minimum or maximum of amplitude varies in response to an environmental stimulus imposed upon the corresponding sensor. The tunable VCSEL individually illuminates each of the sensors throughout the sensor""s associated wavelength band including as examples an amplitude minimum or maximum. A reference fiber length (serving as one arm of the Mach-Zander interferometer) is disposed in a separate optical path from the path connecting said VCSEL assembly and an optical sensor or sensor array. At least one coupler must be provided to divide the optical signal from the tunable VCSEL into the reference arm and sensing paths and to divert the optical signal reflected from the sensor array to another arm of said interferometer. Another coupler must be provided to combine the light from two arms of said interferometer and to direct said combined (interfering) light to the photodetector, the electrical signal from which is relayed to the control block circuitry and external electronic circuitry as required. The control block controls the laser temperature via a thermoelectric element or other means and may or may not adjust the laser power output according to a signal from a monitor photodiode, as required. A tuning controller provides a variable voltage or other signal to the tunable VCSEL indicative of the desired wavelength of the tunable light. A signal processor responsive to the electrical detection signal interprets a shift in the wavelength position of the magnitude minimum or maximum due to the environmental stimulus calculated from the recorded electrical detection signals, and provides a signal indicative of said stimulus.
According to a further exemplary non-limiting illustrative implementation, an optical sensor diagnostic system includes an integrated MEMS-tunable VCSEL for providing a wavelength-tunable light in response to a voltage or other control signal, the tunable light being launched into an optical waveguide. At least one optical sensor, disposed in the path of the tunable light, provides a transmitted light having an associated local amplitude and/or phase perturbation (for example, maximum or minimum). The wavelength at said minimum or maximum of amplitude varies in response to an environmental stimulus imposed upon the corresponding sensor. The tunable VCSEL individually illuminates each of the sensors throughout the sensor""s associated wavelength band including as examples an amplitude minimum or maximum. A reference fiber length (serving as a arm of an interferometer) is disposed in a separate optical path from the path connecting said VCSEL assembly, optical sensor or sensor array and optical detector. One or two reflection means may be disposed in a reference fiber length to create a reference optical length. An optical splitter (for example optical coupler) must be provided to divide optical signal from the tunable VCSEL into the reference and sensing paths. An optical combiner (for example another optical coupler) must be provided and combine the optical signal transmitted through the sensor array and through the reference path and to direct combined light to the photodetector, the electrical signal from which is relayed to the control block circuitry and external electronic circuitry as required. The control block controls the laser temperature via a thermoelectric element or other means and may or may not adjust the laser power output according to a signal from a monitor photodiode, as required. A tuning controller provides a variable voltage or other signal to the tunable VCSEL indicative of the desired wavelength of the tunable light. A signal processor responsive to the electrical detection signal interprets a shift in the wavelength position of the magnitude minimum or maximum due to the environmental stimulus calculated from the recorded electrical detection signals, and provides a signal indicative of said stimulus.
In accordance with a further aspect of an exemplary non-limiting illustrative implementation, an optical sensor diagnostic system includes an integrated MEMS-tunable VCSEL for providing a wavelength-tunable light in response to a voltage or other control signal, the tunable light being launched into an optical waveguide. At least one optical sensor, disposed in the path of the tunable light, provides a transmitted light having an associated local amplitude and/or phase perturbation (for example, maximum or minimum). The wavelength at said minimum or maximum of amplitude varies in response to an environmental stimulus imposed upon the corresponding sensor. The tunable VCSEL individually illuminates each of the sensors throughout the sensor""s associated wavelength band including as examples an amplitude minimum or maximum. A reference fiber length (serving as a arm of an interferometer) is disposed between said VCSEL-assembly and an optical sensor or sensor array or between said optical sensor or sensor array and said detector. One or two reflection means are disposed in a reference fiber length to create a reference optical length in a fiber. The optical signal transmitted through the sensor array and the reference path is directed toward the photodetector, the electrical signal from which is relayed to the control block circuitry and external electronic circuitry as required. The control block controls the laser temperature via a thermoelectric element or other means and may or may not adjust the laser power output according to a signal from a monitor photodiode, as required. A tuning controller provides a variable voltage or other signal to the tunable VCSEL indicative of the desired wavelength of the tunable light. A signal processor responsive to the electrical detection signal interprets a shift in the wavelength of the magnitude minimum or maximum due to the environmental stimulus calculated from the recorded electrical detection signals, and provides a signal indicative of said stimulus.
In accordance with a further illustrative non-limiting implementation, the said optical sensors are of the reflective Bragg grating type. The sensors reflect light, having maxima or minima inside the maxima at different or the same reflection wavelength for each sensor, said sensors varying their spectral positions due to an environmental stimulus, such as strain, pressure, temperature, electrical current or magnetic field imposed thereon.
In accordance with a further exemplary non-limiting illustrative implementation, the said optical sensors are of the transmission Bragg grating type. The sensors transmit light, having minima or maxima inside the minima at a different or the same transmission wavelength for each sensor, said sensors varying their spectral positions due to an environmental stimulus, such as strain, pressure, temperature, electrical current or magnetic field imposed thereon.
In accordance with a further exemplary illustrative non-limiting implementation, the said optical sensors are of reflective etalon type. The sensors reflect light, having maxima, minima or maxima and minima at a different reflection wavelength for each sensor, said sensors varying their spectral positions due to an environmental stimulus, such as strain, pressure, temperature, electrical current or magnetic field imposed thereon.
In accordance with a further exemplary illustrative non-limiting implementation, the said optical sensors are of transmission etalon type. The sensors transmit light, having maxima, minima or maxima and minima at a different transmission wavelength for each sensor, said sensors varying their spectral positions due to an environmental stimulus, such as strain, pressure, temperature, current or magnetic field imposed thereon.
The illustrative non-limiting exemplary implementations described herein provide low cost, workable, practical diagnostic systems which function in cooperation with remote optical fiber sensor systems to measure static and dynamic strain, pressure, temperature, electrical currents and magnetic fields as well as acoustic or vibratory perturbations of items or structures and chemical and biological parameters. The remote sensors may be disposed on structures made of metal, plastic, composite, or any other materials that expand, contract, or vibrate, or the sensors may be embedded within such structures or immersed in liquids or gasses. The implementations also provide a wavelength-tunable VCSEL, tunable smoothly and monotonically, and in particular, linearly or sinusoidally tunable with time. The implementations further provide individual illumination of each sensor, thereby allowing all the tunable VCSEL power to be resident in a single narrow wavelength band at any instant in time. Ultra-fine tuning of tunable VCSELs to a few parts per million will allow another order of magnitude increase in precision due to higher resolution and improved computational methods and statistical processing. The very low mass of the MEMS tuning mechanisms allow very high tuning speeds with very low hysteresis, providing the ability to average out optical noise in the sensor systems with many data points and allowing very close spacing of data in wavelength.